Port the Browser to U2

[bshepherdson]

Includes:

• Porting StackView, BrowserView, UpdateDetailView and more.
• Fixes several contextualizing bugs in U2 infrastructure.
• Adds CSS shorthand support to the U2 element parser.
• Adds min/preferred/max widths to Element (used by StackView).

[bshepherdson]

PTAL. I modernized all the MD views to use dynamic instead of on$, and enableCls where appropriate.